Field of the Invention
The invention relates to a device for compensating for deviations in register in a printed product which, more particularly, is produced in a sheet-fed rotary printing press having a plurality of printing units.
Register errors appear in a multicolor printed product if the color separations in the various printing units are not transferred with proper coincidence or screening to the printed product or, in other words, in accordance with the screen pattern in the reproduction or repro-phase. The main source of error for deviations in registration is due to a relative shift in position of the printing plates on the plate cylinders of the individual printing units. To enable deviations in register between the various printing units to be compensated for, register adjusting devices are typically provided on the plate cylinders to permit the position of the printing plates in the circumferential and lateral direction to be corrected.
Deviations in register become negatively apparent in the printed image in two ways: first, they make the printed image blurry, and second, they cause changes in the color of the printed image.
In the final analysis, the quality of a printed product is judged by the human eye. The human eye has a very high capacity for resolution in terms of color variations. Even deviations in register of the order of magnitude of 10 .mu.m result in intolerable color variations in the printed image. This numerical value offers some impression of the demands that effectively functioning register control and adjusting devices in printing presses must satisfy.
Register errors in the printed image cannot be solely ascribed, however, to incorrect positions or settings of the printing plates in the individual printing units. As a function of such parameters as the nature of the printing material to be printed on, dampening, pressure in the printing gap, and so forth, register errors, i.e., so-called narrower or shrunken printing and rounded or swollen printing, appear, especially in sheet-fed offset.
So-called shrunken printing occurs due to a stretching of the damp sheet in the printing nip. As a result, the printed image transferred to the sheet becomes wider from printing unit to printing unit. In order words, the shrunken printing is caused by various factors of moisture and pressure in the individual printing units. In particular, succeeding printing units print seemingly narrower in relation to the preceding printing units.
By so-called swollen printing, there is understood to be an appearance in the sheet offset print that a line extending vertically to the printing direction, which is printed in a printing unit of a sheet-fed offset printing press and thus transferred to the printed product, is not parallel to a corresponding line which was printed in a succeeding printing unit, but rather, has a round or curved course, with a maximum deviation in the middle of the sheet. The cause for the swollen printing resides in a sagging or bending of the gripper systems of the paper-guiding cylinders. Because the gripper systems and cylinders are supported laterally, they experience maximum deflection of the bending line thereof in the middle of the printing press. This effect becomes especially serious in half-revolution cylinders, i.e., in cylinders with twice or double the conventional circumference of printing-unit cylinders because, with such cylinders, the extent of sagging of the respective cylinder, and the concave or convex deformation of the printed product associated therewith, has double the effect.
If the many transfer points in a multicolor printing press are taken into account, it becomes clear what great significance can be ascribed to devices which are capable of compensating for deviations in register caused by so-called shrunken and swollen printing.
From the published German Patent Document DE 41 19 824 C1, an especially constructed cylinder has become known which compensates for the sagging of the cylinder caused by its own weight, and the compressive strain and the drive forces applied thereto. In particular, the cylinder is constructed so that slight sagging can be corrected by an adjusting device. Correction of the leading edge of cylinder-guided sheets in the concave or convex direction can be effected both retroactively and while the printing press is in operation.
In the case of a further sheet compensator which has become known heretofore, a gripper bar is of multipartite construction, and the various parts thereof are positionable individually. With gripper bars having this type of construction, not only a convex or concave deformation but also shrunken printing on a printed product printed in the press can be compensated for. Typically, adjustment of the gripper bars is effected by trial and error, that is, attaining the OK state is greatly dependent upon the skill of the pressman.